The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

3096. 'And ran again to the well' means a lower affection for truth. This is clear from the meaning of 'a well' as truth, dealt with in 2702, but truth that is lower. And as the subject here is the introduction of truth, a lower affection for truth is meant, as stated just above in 3094. For the difference in meaning in the internal sense between 'a spring' and 'a well' see the paragraph just referred to, where it is explained that the expression 'a spring' is used in reference to purer and higher truth, but 'a well' in reference to truth that is not so pure and is lower, as in the present chapter also where 'a spring' is used at one point, 'a well' at another. Natural truth is a lower variety of truth, and the affection for natural truth a lower kind of affection for truth. This truth enables general facts to receive light most nearly, and that enlightenment penetrates further and affects more deeply, see 3094.